Industrial gas turbine engines, such as those used for electrical power generation or as industrial powerplants, are subject to stringent regulation of exhaust emissions, particularly nitrous oxides (NOx), carbon monoxide (CO) and unburned hydrocarbons. In order to minimize undesirable exhaust emissions, industrial gas turbines are equipped with premixing fuel injectors in which fuel and air are thoroughly premixed prior to being discharged into the engine combustion chamber and burned. The thorough premixing of fuel and air ensures a uniformly low flame temperature, which is a prerequisite for suppressing NOx formation, and promotes complete combustion.
One type of premixing fuel injector is a tangential entry injector, examples of which are described in U.S. Pat. Nos. 5,307,634, 5,402,633, 5,461,865 and 5,479,773, all of which are assigned to the assignee of the present application. These injectors feature an annular mixing chamber radially bounded by an axially extending centerbody and a pair of circular arc scrolls. The scrolls are radially offset from each other to define a pair of intake slots, each of which admits a stream of air tangentially into the mixing chamber. Each scroll includes an array of fuel injection passages for introducing fuel into the incoming airstream. The air and fuel enter the mixing chamber, swirl around the centerbody and become intimately intermixed. The fuel-air mixture flows axially through the mixing chamber and discharges into the engine's combustion chamber where it is ignited and burned. Because the tangential entry injector produces a highly uniform, thoroughly blended fuel-air mixture, the injector is exceptionally effective at suppressing NOx formation and promoting complete combustion.
Premixing fuel injectors are called upon to exhibit a number of desirable operational characteristics in addition to thorough fuel and air mixing. For example, a premixing injector should promote spatial and temporal stability of the flame in the combustion chamber. Without such stability, the combustion chamber will be exposed to low frequency pressure oscillations that can stress the combustion chamber, reducing its useful life. In addition, a premixing fuel injector should be internally flame resistant. That is, the injector should resist ingestion of the combustion flame into the mixing chamber and quickly disgorge any flame that overcomes the ingestion resistance. Internal flame resistance is important because combustion inside the mixing chamber can easily damage the scrolls and centerbody, all of which have a limited tolerance for exposure to high temperatures.
Unfortunately, the requirements of thorough fuel-air mixing, flame stability in the combustion chamber, and internal flame resistance are often in conflict. Design features that improve one of these desirable attributes often compromise one or more of the others. Accordingly, achieving an effective combination of thorough fuel and air mixing, good flame stability, and internal flame resistance is a considerable challenge. A fuel injector having good internal flame resistance is described in copending, commonly owned patent applications Ser. No. 08/771,408, now U.S. Pat. No. 5,899,076, and Ser. No. 08/771,409, now U.S. Pat. No. 5,896,739 both filed on Dec. 20, 1996. The disclosed injector has a centerbody contoured so that the fuel-air mixture flows axially at a velocity high enough to resist flame ingestion and to promote flame disgorgement. Another exemplary injector is described in commonly owned copending patent application Ser. No. 08/991,032 filed on Dec. 15, 1997. The disclosed injector features a fuel passage array configured to inject fuel nonuniformly along the length of the air intake slot to improve the uniformity of the fuel-air mixture and thereby suppress undesirable exhaust emissions. The injector also features a centerbody having a bluff surface perpendicular to the injector axis, and axially aligned with the injector discharge plane. In operation the combustion flame tends to remain anchored to the bluff surface, improving the flame's spatial stability and discouraging the flame from migrating into the mixing chamber.
Despite the many merits of the above described injectors, engine manufacturers continue their efforts to perfect and optimize premixing fuel injectors. In particular, manufacturers seek additional ways to eliminate flame from the mixing chamber without increasing exhaust emissions or jeopardizing flame stability.